This invention relates in general to radio signal receiving systems of the space diversity type and more particularly to a frequency offset receiver utilizing differential detection.
One of the difficulties in UHF and VHF mobile radio environments, such as satellite systems, is that line of sight communications rarely occur. A received radio signal is typically the vector sum of reflections from near-by objects and is therefore subject to rapid and deep multipath fading as the receiver moves. In order to realize a high quality mobile radio system, it is important mitigate the communication degradation caused by this fading.
Various techniques have been utilized in the past to overcome the problem of multipath fading. One such successful technique is known as diversity, wherein two or more independent samples of a transmitted signal are received. Frequency, time, and space diversity have all been successfully employed to overcome the multipath fading problem. Frequency and time diversity are accomplished by retransmitting information on a separate frequency or at a delayed time, respectively, requiring considerable bandwidth and power to implement. Space diversity, on the other hand, simply requires two or more signal receiving antennas spaced sufficiently apart so that their fading patterns are independent. It has been shown that space diversity yields better reception than frequency or time diversity, and is the preferred technique where channel bandwidth and power are constrained, such as in satellite systems.
Three common techniques have been employed for combining the received signals from multiple antennas of a space diversity system; selection combining, maximal-ratio combining, and equal gain combining.
Selection combining is a technique for selecting, in real time, the best signal among a plurality of signals received from respective antennas. According to this technique, the strongest signal is accepted and the weaker signals are rejected. The selection combining technique is simpler to implement than maximal-ratio or equal gain combining techniques, but is inferior in terms of performance gain. In order to avoid switching noise, it is necessary to perform switching subsequent to demodulation, requiring expensive parallel complete receiving systems. Also, complex and expensive signal quality detection and switching controller circuits are typically required.
According to the equal gain combining technique, intermediate frequency (IF) signals are generated having equal frequencies and phase such that the IF signals can be combined in phase and at the same relative level as they are received. The combined output signal is monitored to provide automatic gain control of the IF amplifiers in each of the receiving branches, to ensure a constant amplitude. However, this technique requires complex carrier phase adjustment of each signal or, alternatively combination of the signals after demodulation via parallel complete receiving systems.
The maximal-ratio combining technique affords the best results in signal reception reliability. This technique is similar to equal gain combining except for the method of controlling the gain for each IF signal. Equal gain combining requires that the relative gain for each IF signal be the same, whereas maximal-ratio combining requires that the gain for each IF signal be proportional to the received signal level itself. A weaker signal is controlled to contribute a proportionally smaller amount of itself to the resultant common IF output signal, than does a stronger signal. Thus, it is necessary to measure the signal-to-noise ratio of each branch and to change the gain for each branch instantaneously, in order to get the maximum signal-to-noise ratio by combining them. It is also necessary to adjust the carrier phase of each signal if the signals are combined before demodulation. In order to remove this complex phase adjuster, the signals can be combined after demodulation, as in the case of equal gain combining, but in this case parallel complete receiving systems are also required.
A transmitter diversity technique has been disclosed in an article entitled A TRANSMITTER DIVERSITY FOR MSK WITH TWO BIT DIFFERENTIAL DETECTION, by Murota et al., reprinted from the IEEE International Conference on Communications, ICC 1982, June 13-17, 1982, Philadelphia, Pa. According to this article, two signals are received by a single antenna. Their multipath fading patterns are independent, resulting in improved signal detection in the event of multipath fading. However, the two signals are also subjected to shadowing, resulting in decreased reception quality. Also, the technique of Murota et al. suffers from the disadvantage of requiring two separate radio channels, which is a disadvantage in most mobile radio systems wherein channel bandwidth is constrained.